System and method for receiver sensitivity improvement

ABSTRACT

A system and method for ultrashort signal detection adds an optical weighting element upstream of a detector within a direct detection receiver. The optical weighting element is configured to generate an optical pulse that closely matches at least one ultrashort pulse within the input signal so that portions of the input signal that are nonoverlapping with the at least one ultrashort pulse are rejected.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 16/629,022, filed Jan. 6,2020, issued as U.S. Pat. No. 11,212,011, which is a 371 national stagefiling of International Application No. PCT/US2018/041123, filed Jul. 6,2018, which claims the benefit of the priority of U.S. Application No.62/530,034, filed Jul. 7, 2017, each of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention is directed to a direct detection receiver schemewith improved sensitivity for detection of a wideband signal obscured byoptical noise.

BACKGROUND OF THE INVENTION

Wide bandwidth, short pulse duration, and highly correlated frequencycontent of femtosecond and picosecond optical pulse sources have givenrise to technological advances in a variety of disciplines, includingtelecommunications, medical imaging and treatment, biology, chemistry,and metrology. The key factor that contributes to wide deployment ofultrafast pulse sources—the short pulse duration—also determines thefrequency span of ultrafast events. While significant progress has beenmade in ultrafast pulse source design and performance, the associateddetector technology has failed to keep pace. As a result, thereliability of information carried by an ultrafast signal is defined,and limited, by detector performance.

In general, an ideal detector maximizes signal energy while overcominguncertainty introduced by noise—this characteristic becomes even moreimportant for short optical pulses. Optimal signal-noise discriminationis realized if the detector impulse response matches the incomingoptical signal, an approach referred to as “matched filtering,” which isthe optimal linear filter for maximizing the signal-to-noise ratio (SNR)in the presence of additive stochastic noise. The optimal detection ofan ultrashort, wide-band signal, dictates a wide-band (˜THz) detector,well beyond the capabilities of existing detector technology which islimited to a range of less than 100 GHz, thus making optimal directdetection of femtosecond and picosecond optical pulses unattainable atthis time. The signal detection task becomes significantly morechallenging if an optical signal is buried in noise, a scenarioparticularly common in telecommunication, medicine, biology, andastronomy. Sub-optimal detection of the optical signal obscured by noisedeteriorates detected signal quality, often resulting in informationloss. The electrical bandwidth parameter, therefore, sets the limit onthe detected signal fidelity, posing the fundamental limitation inultrafast signal reception and decoding.

Various techniques have been attempted to decrease signal estimationuncertainty, including time averaging, spectral slicing, and timegating. When applied to ultrafast pulse detection, each approachmanifests a distinct set of trade-offs. Time average represents thesimplest way to reduce noise, but has the disadvantage of decreasingmeasurement speed, which is detrimental in most applications. Spectralslicing has been used for signal event detection, however, itsimplementation in sub-picosecond pulse detection raises the problem thata direct-filtering realization is prone to pulse distortions. Thealternative multicasting approach requires large frequency comb spectralwidths. The time gating technique is arguably the most widely useddetection method in femtosecond pulse detection, but its use is limitedto applications where the signal timing is known. Despite the associatedcomplexity, time gating has been implemented in coherent receiverarchitecture to eliminate unwanted temporal events or noise innarrow-band pulses, or to reduce timing jitter and inter-symbolinterference effects in direct detection receivers. Althoughwidely-used, the inherent relation that exists between ultrafast timegating and narrow-band optical-to-electrical receiver performance indetection of noisy ultrafast signals has not been addressed in real-timesystems to date.

The detected signal quality represents one of the central limitations inthe ultrashort pulse source implementation due to inability of current,narrow-band, detector technologies to distinguish between the usefulsignal and integrated noise. The sensitivity of the narrow-band detectorcan be improved if its performance is decoupled from the electricalbandwidth.

SUMMARY

To address the limitations in the prior art, the inventive approachemploys an optical gate prior to a narrow-band detector and demonstratea receiver sensitivity enhancement, quantified by an order of magnitudedecrease of the probability of error. Receiver sensitivity can bemarkedly improved with the aid of a signal-matched optical gate thatmitigates the sub-optimal detector deficiencies.

The operating principle of an optical weighting receiver according toembodiment of the invention involves applying opticalweighting—multiplying by a weighting function (ƒ(t)) prior to impulseresponse detection. The optical weighting operation rejects opticalnoise beyond the signal temporal duration, thus preempting the mixing ofthe signal with the portion of the noise that does not overlap in timewith signal.

In one aspect of the invention, an optical receiver for ultrafast signaldetection includes a direct detection receiver configured for receivingan input signal, the input signal comprising at least one ultrashortpulse within signal noise; and an optical weighting element disposedupstream of the direct detection receiver, the optical weighting elementconfigured to impose optical weighting on the input signal to excludeone or more portions of the input signal that are nonoverlapping in timewith the at least one ultrashort pulse. The optical weighting may beconfigured to exclude a period of time within the input signalcorresponding to the at least one ultrashort pulse. The modulator may beconfigured to implement an optical weighting function ƒ(t) according tothe relationship

${f(t)} = \left\{ {\begin{matrix}{{s(t)},{{t_{0} - \frac{\tau_{s}}{2} + {nT}} < t < {t_{0} + \frac{\tau_{s}}{2} + {nT}}}} \\{0,\ {otherwise}}\end{matrix}.} \right.$In some embodiments, the optical weighting element is an amplitudemodulator. In other embodiments, the optical weighting element is a fourwave mixer. In another aspect of the invention, an optical receiver forultrashort signal detection includes an optical weighting elementdisposed upstream of a detector within a direct detection receiver, theoptical weighting element configured to generate an optical pulse thatclosely matches at least one ultrashort pulse within the input signal sothat portions of the input signal that are nonoverlapping with the atleast one ultrashort pulse are rejected. The optical weighting elementmay execute an optical weighting function ƒ(t) according to therelationship

${f(t)} = \left\{ {\begin{matrix}{{s(t)},{{t_{0} - \frac{\tau_{s}}{2} + {nT}} < t < {t_{0} + \frac{\tau_{s}}{2} + {nT}}}} \\{0,\ {otherwise}}\end{matrix}.} \right.$In some embodiments, the optical weighting element is an amplitudemodulator. In other embodiments, the optical weighting element is a fourwave mixer.

In yet another aspect of the invention, a method for increasingsensitivity for detection of ultrafast optical signals includesinserting an optical weighting element upstream of a detector within adirect detection receiver, wherein the optical weighting element isconfigured to generate an optical pulse that closely matches at leastone ultrashort pulse within the input signal so that portions of theinput signal that are nonoverlapping with the at least one ultrashortpulse are rejected. The optical weighting element may execute an opticalweighting function ƒ(t) according to the relationship

${f(t)} = \left\{ {\begin{matrix}{{s(t)},{{t_{0} - \frac{\tau_{s}}{2} + {nT}} < t < {t_{0} + \frac{\tau_{s}}{2} + {nT}}}} \\{0,\ {otherwise}}\end{matrix}.} \right.$In some embodiments, the optical weighting element is an amplitudemodulator. In other embodiments, the optical weighting element is a fourwave mixer.

The benefit of the new approach, termed “optical weighting receiver”(“OWR”), becomes apparent when contrasted with the performance of theconventional direct-detection receiver in detection of picosecond-long,noisy signal. Specifically, the receiver performance can becharacterized by measuring the error detection probability (P_(e)) andreceiver operating characteristics (ROC). Both measurements confirm theOWR performance superiority: the P_(e) shows a seventeen-fold decrease,while the detection efficiency in the ROC measurement increases by afive-fold. This sensitivity improvement approach opens the door to a newclass of opto-electronic detector technology that can radically improvedetected signal integrity while adding only nominal complexity. Theresulting improvement in performance has the potential to solveheretofore intractable problems that have hampered progress in variousdisciplines.

Presently, the sensitivity of the direct-detection receiver is dependenton the electrical bandwidth of the detector and detection electronics,thus preventing the optimal detection of an ultrashort signal obscuredby noise (picosecond and sub-picosecond long signal). The inventive OWRcan decouple the receiver performance from the electrical bandwidth indetection of the ultrashort signal buried in noise. The result is anorder of magnitude reduction of the Bit Error Ratio measurement of theoptical gate-aided receiver compared to a direct detection receiver.

Potential applications of the invention for improving the receiversensitivity in detection of ultrafast (picosecond and shorter) signalobscured by noise include detection of distant radar signals, detectionof ultrawideband radio signals, detection of navigational signals,ultrafast spectroscopy, radio astronomy, and medical and bio-imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic view of the operating principle of an opticalweighting receiver according to embodiment of the invention comparedagainst a conventional direct detection receiver; FIG. 1B is enlargedview comparing simulated detector outputs of the OWR and DDR of FIG. 1A.

FIGS. 2A-2F illustrate estimated error probability (P_(e)) for simulatedreceiver parameters. FIGS. 2A and 2B illustrate a comparison of theestimated P_(e) for a direct detection receiver (DDR) (FIG. 2A) and anOWR according to the present invention (FIG. 2B). FIGS. 2C and 2D show asimulated comparison of the estimated error probability for narrow-bandDDR and narrow-band OWR, respectively. FIG. 2E illustrates the estimatedP_(e) for an OWR with a narrow-band detector and a constant opticalfilter bandwidth of 300 GHz where the gating pulse is varied between0.8% and 50%. FIG. 2F shows estimated P_(e) for an OWR with anarrow-band detector and a constant optical filter bandwidth of 300 GHzwhere the gating pulse width is comparable to the signal pulse width.

FIGS. 3A and 3B illustrate exemplary experimental setups for a DDR andOWR, respectively.

FIGS. 4A and 4B are plots of P_(e) measurement as a function of opticaland electrical filter bandwidth for a DDR and OWR, respectively.

FIG. 5A is a plot of the receiver operating characteristics (ROC) for aDDR and OWR at different signal powers; FIG. 5B is an enlarged view ofthe area within the circle of FIG. 5B.

FIGS. 6A and 6B illustrate simulation setups of an OWR and DDR,respectively.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Previous theoretical and experimental work relates receiver performanceto three parameters: the optical filter bandwidth, the electrical filterbandwidth, and the signal duty cycle. Matching bandwidth parameters tothe input pulse improves receiver sensitivity for a given optical and anelectrical filter transfer function. However, in many applicationsinvolving picosecond and femtosecond pulses this task remainsunattainable. The inventive approach of decoupling the receiverperformance from the electrical bandwidth in the described setting canbe numerically tested by contrasting direct detection and the OWR schemeand is described herein.

FIG. 1A illustrates the fundamental difficulties in detection of anultrashort signal 102 with a conventional direct detection receiver(DDR) 152 and the improvement obtained with the optical weightingreceiver (OWR) 122 according to the present invention. In FIG. 1A, inputpulse 102 (in frequency domain (ƒ) is shown in the time domain (t) as ashort pulse 112 (E_(signal)) within background noise 114 (E_(noise)),which is incident onto direct detection receiver 152 with an impulseresponse h_(e)(t) 126. After integration 128, the detection current 134,136 is represented as the integral of the sum of the signal and thenoise power over the detector impulse response time h_(e)(t). In theideal (matched-detection) case, the electrical weighting function 126matches the signal and only the noise that coincides with the signalreaches the integrator 128 and the noise is weighted exactly by thesignal profile. In practice, however, the conventional electricalweighting function h_(e)(t) (direct-detection case, in the time domain)is much longer than the ultrashort signal, mandating accumulation ofexcess noise much beyond the signal time extent, and loss of detectionfidelity.

For the optical weighting receiver 122, the short pulse 112 hidden innoise 114 is optically gated prior to detection 124 (ƒ(t)). Opticalweighting 124 rejects optical noise beyond the signal temporal duration,thus preempting the mixing of the signal with the portion of the noisethat does not overlap in time with signal to yield pulse 104 in thefrequency domain, pulse 106 in the time domain. In practical terms, thenew receiver circumvents the electrical bandwidth limitation necessarilyimposed on photon-electron conversion by rejecting the noise beforehand.The remaining operations of squaring and electrical weighting (impulseresponse h_(e)(t)) 126 and integration 128 produce detection current130, 132, with vastly improved discrimination. FIG. 1B provides anenlarged image of the resulting detection currents for the DDR (134,136) and OWR (130, 132) to clearly show the enhanced signal within thenoise.

The detection process is represented as the convolution of the opticalpower with the detection system impulse response. The result of thephoton-to-electron conversion in the direct detection process is thecurrent I_(dd)(t), typically represented as a product of theresponsivity parameter, R, and a convolution of the optical power,|E(t)|² with the detection system impulse response h(t):

$\begin{matrix}{{I_{dd}(t)} = {{R{\int_{- \infty}^{\infty}{{{{E_{s}(\tau)} + {E_{n}(\tau)}}}^{2}{h\left( {\tau - t} \right)}d\;\tau}}} = {{R{\int_{- \infty}^{\infty}{{{E_{s}(\tau)}}^{2}{h\left( {\tau - t} \right)}d\tau}}} + {R{\int_{- \infty}^{\infty}{\left( {{{E_{n}(\tau)}}^{2} + {2{{Re}\left( {{E_{s}(\tau)}{E_{n}^{*}(\tau)}} \right)}}} \right){h\left( {\tau - t} \right)}d\tau}}}}}} & (1)\end{matrix}$and |E(t)|=|E_(s)(t)+E_(n)(t)|, where E_(s,n) are the signal and noisefields respectively. The OWR detection process can be mathematicallyrepresented in the following manner:I _(OWR)(t)=R∫ _(−∞) ^(∞) ∥E _(s)(τ)h _(g)(τ)+E _(n)(τ)h_(g)(τ)|h(τ−t)dτ=R∫ _(−∞) ^(∞) |E _(s)(τ)|² h _(g)(τ)h(τ−t)dτ+R∫ _(−∞)^(∞)(|E _(n)(τ)|²+2Re(E _(s)(τ)E _(n)*(τ)))h _(g)(τ)h(τ−t)dτ   (2)where ƒ(t) is the optical weighting function, given as

$\begin{matrix}{{f(t)} = \left\{ {\begin{matrix}{{s(t)},{{t_{0} - \frac{\tau_{s}}{2} + {nT}} < t < {t_{0} + \frac{\tau_{s}}{2} + {nT}}}} \\{0,\ {otherwise}}\end{matrix},} \right.} & (3)\end{matrix}$where τ_(s) is the weighting function duration, to is the pulse center,T is the pulse period, and n is an integer. In the ideal(matched-detection) case, the electrical weighting function matches thesignal and only the noise that coincides with the signal reaches theintegrator and the noise is weighted exactly by the signal profile. Inpractice, however, the conventional electrical weighting function in thedirect detection case, in the time domain, is much longer than theultrashort signal, resulting in the accumulation of excess noise beyondthe signal time duration, severely degrading the detected signalfidelity.

The inventive approach deals with the noise addition problem byrejecting optical noise outside of the signal time span, beforedetection. In an exemplary embodiment, this method is implemented byadding optical weighting to reject optical noise beyond the signaltemporal duration to avoid mixing of the signal with the portion of thenoise that does not overlap in time with the signal. In practical terms,the OWR circumvents the electrical bandwidth limitation that isnecessarily imposed on photon-electron conversion by rejecting the noisebeforehand.

The benefits of the OWR technique were first modeled and compared todirect detection in detection of an ultrashort signal represented by aGaussian 1.5 ps long noisy signal repeating over a 100 ps period.Detected signal fidelity was characterized by calculating theprobability of error (P_(e)), using ta Bit Error Ratio (BER) procedurederived from well-established telecommunication characterizationmethods. Specifically, the widely-used system characterization approachdescribed by Winzer and Kalmar (“Sensitivity enhancement of opticalreceivers by impulsive coding”, Journal of Lightwave Technology 17(2),171-177 (1999)) was adopted to test the probability of error (P_(e))driven by two physical parameters: optical filter bandwidth andelectrical filter bandwidth. The evaluation was performed usingcommercial simulation software from VPIphotonics Inc. (Norwood, Mass.).Numerical simulations were performed using the setups shown in FIGS. 6Aand 6B, representing the OWR and DDR, respectively. The OWR receiverperformance was tested by detecting the 1.5 ps signal 602 with 10 GHzrepetition rate. Signal pulses with 0.14 mW peak power were amplitudemodulated by amplitude modulator (AM) 604 with a pseudo-random bitsequence 606 and combined with white noise from optical noise source(ONS) 608 with power spectral density of 1.92×10⁻¹⁷ W/Hz. The resultingoptical-signal-to-noise-ratio of 0.3 is estimated at operation 609 bymeasuring the average signal and the noise power independently. Thecombined fields are next passed through a Gaussian-shapedoptical-band-pass-filter (OBPF) 610 and either detected directly atdetector 614 or optically gated at amplitude modulator 612 with 1.2 pspulses before detection. Pulse detection was performed by the square-lawdetector and both a dark current of 0.1 nA and shot noise were addedfrom electrical noise source (ENS) 616 in the detection process. Thetotal current is passed through a fourth-order Bessel-shaped low-passfilter (LPF) 618 with bandwidth Be and guided to the P_(e) test engine620. The DDR shown in FIG. 6B has a similar architecture to that of theOWR of FIG. 6A, with the main difference being the absence of theamplitude modulator 612 that introduces optical weighting before thedetector 614. Both schemes were tested under equivalent conditions.

The inventive approach to achieving optical weighting of the inputsignal is not limited to use of the amplitude modulator (AM) arrangementas in the example shown in FIG. 6B. In fact, the experimental set-updescribed with reference to FIG. 3B, described in more detail below,utilizes a four-wave mixing (FWM) process within a highly nonlinearfiber (HNLF) to generate an idler capable of achieving the appropriateoptical weighting. The FWM approach may actually be preferable to the AMmethod for speed purposes. Selection of other high speed opticalprocesses for generating a matched weighting pulse will be apparent tothose of skill in the art.

Results of the simulations are shown in FIGS. 2A-2F. FIG. 2A representsthe ideal, wide-band DDR with a minimum P_(e) of 0.00012. For purposesof this description, this value is considered as optimal. The P_(e)associated with the OWR scheme (FIG. 2B) also reaches the optimal P_(e)level. However, in a more realistic setting, employing a narrow-banddetector, the DDR performance deteriorates notably. This scenariobecomes apparent in FIGS. 2C-2D, where the minimum P_(e) achieved withthe OWR method remains an order of magnitude lower than that of the DDR.Further, the OWR performance remainselectrical-filter-bandwidth-independent, allowing the scheme to attainthe optimal P_(e) magnitude with the narrow-band receiver. Note that inthe absence of optical weighting, no amount of electrical filtering,post square-law detection, can bring the performance below the level of10⁻³.

To evaluate the OWR's performance dependence on the gating pulse width,the electrical filter bandwidth was varied in the 10 GHz to 40 GHz rangeand the gating pulse width 0.8 to 32 times that of the input pulse. Asexpected, the OWR performance depends strongly on the gating pulsewidth, demonstrated by significant P_(e) variations depicted in FIGS.2E-2F. (Note that FIG. 2F is an expansion along the y-axis of the areawithin the black rectangle at the bottom of FIG. 2E.) Plotted P_(e) mapspoint to an optimal optical weighting design—matching the weightingpulse to the signal directly optimizes the receiver performance.

Example 1—Experimental Verification

The experimental setup for the verification of the benefits of theinventive OWR approach, emulating the standard telecommunicationpre-amplified receiver architecture, is diagrammatically illustrated inFIGS. 3A and 3B for a DDR and OWR, respectively.

Probability of error (P_(e)) measurement was performed using signalpulses 304 of 1.5 ps temporal width and 10 GHz repetition rate, centeredat 1557.4, having sinusoidal amplitude and phase modulation, produced bypulse source 302 and compressed in a dispersive fiber 306. The pulsesource 302 was divided to create the signal and the weighting pulsesource 302′ (seen in FIG. 3B). The signal was amplitude modulated atamplitude modulator 308 with a 2⁷-1-long pseudo-random bit sequence 310and then frequency converted to 1544 nm in the highly nonlinear fiber(HNLF) by means of the four-wave mixing process. The HNLF wascharacterized by the following global parameters: 182 m fiber length,1555 nm average zero dispersion wavelength, 0.029 ps/(nm² km) dispersionslope and 28 W⁻¹km⁻¹ nonlinear parameter. The signal was next attenuatedat attenuator 312 to 24.2 nW and then amplified in an Erbium-doped fiberamplifier (EDFA) 314. The signal average power is monitored at the EDFAinput. The value reported in this measurement corresponds to the powerbefore the amplifier front loss—this loss is 0.5 dB (0.5 dB insertionloss corresponds to the loss of two components before the Erbium fiber:the wavelength division multiplexer for combining the pump and signal,and the isolator. The splice loss is not taken into account. Thebandwidth of optical bandpass filter (OBPF) 316 at the amplifier outputis varied, having the following values: 0.6 nm, 1 nm, 2 nm, 5 nm and 7nm.

In case of the optical weighting receiver of FIG. 3B, the amplifiedsignal is filtered at filter 316 after which a weighting pulse isgenerated in an optical fiber using a four-wave mixer (FWM). In thisapproach, the filtered signal is combined with the weighting pulse fromsource 302′ at combiner 330, which acts as the pump wave 350, thenguided into a 20 m long HNLF 320. The generated idler, centered at 1571nm, is then filtered using a 20 nm OBP filter 321 to remove theweighting pump and the signal. The idler 332 is then detected using a 40GHz photodiode 318, followed by a 33 GHz electrical amplifier 322, alow-pass filter 324 and an acquisition system 326, which in the testset-up was a real-time oscilloscope. The electrical filter bandwidth isvaried at the oscilloscope, having the following set of values: 11 GHz,20 GHz, 25 GHz, 30 GHz, and 33 GHz. The sampling rate of the real-timeoscilloscope was set to 100 GS/s and 10⁵ points are collected peracquisition, corresponding to 10⁴ bits. The reported P_(e) value isaveraged over six measurements.

In the direct detection case, the amplified signal is detected directlyby the same detection system as described above. In both the directdetection receiver scheme and the OWR scheme, a single polarization isdetected at the amplifier output, by adding a polarization beam splitterafter the amplifier. Both measurements are performed under the sameconditions and the P_(e) averaged over six measurements. The P_(e) mapis represented in FIGS. 4A and 4B for the DDR and OWR architectures,respectively.

Measurements represented in FIGS. 4A and 4B confirm the significantimprovement provided by the OWR approach. As expected, the OWR schemeoutperforms the standard direct detection receiver, demonstrated by theseventeen-fold improvement in the P_(e) measurement. Specifically, aminimum P_(e) of 0.0032 is measured with the DDR scheme, while an orderof magnitude lower minimum P_(e) of 0.00018 is assessed with the OWRscheme under equivalent conditions. Still, some electrical filterbandwidth dependence is observed in the OWR measurement, a trendcontributed to the sub-optimal gating pulse width and the finite gatingpulse extinction ratio.

Finally, the two detection schemes are contrasted by measuring thereceiver operating characteristics (ROC) curve at different averagesignal power levels <P_(s)>. FIGS. 5A and 5B represent the dark countprobability, measured as a function of the detection efficiency byvarying the detection threshold. Measurements were performed for adifferent average signal power at the amplifier input, ranging from 1.54nW to 23.99 nW (corresponding to 1.2 to 18.6 average number of photonsper bit). In the ROC plots, the DDR results are represented by the “º”marker while the OWR results are represented by “*” marker. The curvepairs for the DDR and OWR are numbered from right to left as indicatedin Table 1 below:

Curve Pair No. Signal Power <P_(s)> 1 23.99 nW 2 12.19 nW 3  6.15 nW 4 1.54 nWFIG. 5B is an enlargement of the area in the circle within FIG. 5A,showing the detailed results for the 1.54 nW signal power (curve pairno. 4).

The dark count probability is measured as the probability of sending thelogical “zero” state and detecting the “one” state, P(1|0); thedetection efficiency is measured as the probability of sending thelogical “one” state and detecting the logical “one” state, P(1|1). Inthis measurement, the sampling rate was set to 100 GS/s, as in the caseof the P_(e) measurement. The number of bits per acquisition was 10⁴ andwas chosen for computational expediency. The ROC curves are assembled bytaking multiple single-shot measurements at distinct points in time toincrease the total number of points. The total number of points wasgreater than 10⁵.

The average photon number per bit is estimated from the average power,measured at the amplifier input. The average photon number per bit<N_(bit)> of the classically attenuated signal is calculated with thefollowing equation:

$\left\langle N_{bit} \right\rangle = \frac{\left\langle P \right\rangle T}{hf}$where <P> is the average signal power, T is signal period, h is Planck'sconstant and ƒ is signal frequency. The average photon number per pulseis twice the average photon number per bit, where the factor of twooriginates from the PRBS modulation, having the same probability of 0and 1.

These measurements again clarify superiority of the inventive OWRapproach, demonstrated by more than 5-fold detection efficiency increasefor a fixed dark count probability of 0.001 and average signal power atthe amplifier input of <P_(s)>=1.54 nW. The OWR scheme showsunprecedented performance in direct detection of ultrashort signalobscured by noise. These results represent an important milestone inoptimizing ultrashort signal detection.

The OWR approach described herein has been shown to outperform a typicalnarrow-band receiver in ultrashort pulse detection. Specifically,experimental results demonstrate the viability of decoupling thereceiver performance from its electrical bandwidth in the OWRarchitecture, allowing the scheme to reach the optimal P_(e) value withnarrow-band receiver. Experimental measurements are consistent withtheoretical predictions; seventeen-fold improvement in the P_(e) isachieved with the OWR scheme, compared to a direct detection scheme.Reconcilable results are observed in the ROC measurement, where morethan 5-fold increase in the detection efficiency is evaluated for theaverage input signal power of 1.54 nW at the amplifier input.

The inventive OWR approach shows immense potential in achieving optimalultrafast pulse detection, with a plethora of applications acrossdisciplines. Specifically, besides typical telecommunication receiverarchitecture demonstrated in this work, many applications in medicineand biology could largely benefit from the OWR receiver. Recentbreakthroughs in histopathological tissue imaging provide one example ofsuch an application that is limited by its reliance on existing directdetection receiver architecture for ultrafast pulse detection. Opticalcoherence tomography-based imaging is another example of an applicationin which a coherent detection scheme is implemented for ultrashort pulsedetection. While coherent detection technique is inherently moresensitive than the direct detection method, it is also a subject tostrict arm balancing requirements that are often difficult to meet. TheOWR scheme, thus, has potential to improve the detection sensitivity ofthe misbalanced coherent receiver. Such improvement could significantlyincrease the acquisition rate and possibly the depth range in opticalcoherence tomography.

In summary, receiver sensitivity improvement is achieved by employingoptical weighting prior to detection of the picosecond-long signalobscured by noise. Findings indicate the possibility of decouplingreceiver performance from its electrical bandwidth, ultimatelyoptimizing the probability of error. Such sensitivity improvement isbeyond the reach of the standard, narrow-band, direct detection receivertechnologies. Although the preceding description involves sensitivedetection of picosecond-long pulses, the technique applies to pulses ofarbitrary duration. Indeed, the technological advancement represented bythe OWR approach provides a path to a new class of opto-electronicdetector technology with a potential to bring profoundly transformativeeffects across many disciplines.

As will be apparent to persons of skill in the art, modifications andvariations to the specific detector types and architectures described inthe examples may be made without deviating from the inventive approachof introducing an optical weighting element and operation prior todetection of ultrafast signals. Accordingly, the scope of the inventionis not intended to be limited to the specific embodiments describedherein, but is intended to encompass all optical detector arrangementsthat employ optical weighting to preempt mixing of noise within theinput signal with the portion of the signal corresponding to theultrashort pulse.

REFERENCES

-   [1] Khilo, A., et al., “Photonic ADC: overcoming the bottleneck of    electronic jitter”, Optics Express 20(4), 4454-4469 (2012).-   [2] Koos, C., et al., “All-optical high-speed signal processing with    silicon-organic hybrid slot waveguides”, Nature Photonics 3, 216-219    (2009).-   [3] Kuo, B. P.-P., et al., “Transmission of 640-Gb/s RZ-OOK channel    over 100-km SSMF by wavelength-transparent conjugation”. Journal of    Lightwave Technology 29(4), 516-523 (2011).-   [4] Sibbett W., et al., “The development and application of    femtosecond laser systems,” Optics Express 20(7), 6989-7001 (2012).-   [5] Konig, K, et al., Clinical two-photon microendoscopy,”    Microscopy Research and Technique 70, 398-402 (2007).-   [6] Barretto, R. P. J., et al., “Time-lapse imaging of disease    progression in deep brain using fluorescence microendoscopy,” Nature    Medicine 17, 223-228 (2011).-   [7] Mejan, G. et al., “Remote detection and identification of    biological aerosols using femtosecond terawatt lidar system,”    Applied Physics B 78, 535-537 (2004).-   [8] Udem, T. H., et al., “Optical frequency metrology,” Nature 416,    233-237 (2002).-   [9] McDonoigh, R. N.; and Whalen, A. D. Detection of signals in    noise (Academic Press, San Diego, 1995).-   [10] Kay, S. Fundamentals of statistical signal processing, Volume    II: Detection theory (Prentice Hall, Upper Saddle River, 1998).-   [11] Winzer, P. J.; and Kalmar, A., “Sensitivity enhancement of    optical receivers by impulsive coding,” Journal of Lightwave    Technology 17(2), 171-177 (1999).-   [12] Chen, Y., et al., “Reducing ASE effect in coherent detection by    employing double-pass fiber preamplifier and time-domain filter,”    IEEE Journal of Quantum Electronics 45(10), 1289-1296 (2009).-   [13] Sanjoh, H., et al., “Time-domain filtering to reduce    signal-spontaneous beat noise for a pulse-type multiwavelength light    source,” IEEE Photonics Technology Letters 15(5), 757-759 (2003).-   [14] Derickson, D.; and Muller. M. Digital Communications Test and    Measurement (Prentice Hall, Boston, 2008).-   [15] Slavik, R., et al., “All-optical phase and amplitude    regenerator for next-generation telecommunications systems.” Nature    Photonics 4, 690-695 (2010).-   [16] Otsuji, T., et al., “10-80-Gb/s highly extinctive electrooptic    pulse pattern generation,” IEEE Journal of Selected Topics in    Quantum Electronics 2(3), 643-649 (1996).-   [17] Andrekson, P. A.; and Westlund, M. Nonlinear optical fiber    based high resolution all-optical waveform sampling. Laser &    Photonics Reviews 1(3), 231-248 (2007).-   [18] Agrawal, G. P. Nonlinear Fiber Optics (Academic Press, San    Diego, 2007).-   [19] Osche, G. R. Optical detection theory for laser applications    (Wiley, Hoboken, 2002).-   [20] Tu, H, et al., “Stain-free histopathology by programmable    supercontinuum pulses,” Nature Photonics 10, 534-540 (2016).-   [21] Huang, D., et al., Optical coherence tomography. Science 254,    1178-1181 (1991).

The invention claimed is:
 1. An optical receiver for ultrafast signaldetection, the receiver comprising: a direct detection receiverconfigured for receiving an input signal generated by a pulse source,the input signal comprising at least one ultrashort pulse within signalnoise, the at least one ultrashort pulse having a temporal duration; anoptical pulse generator configured to generate an optical weightingpulse that closely matches the at least one ultrashort pulse; and anoptical bandpass filter configured to receive a combination of the inputsignal and the optical weighting pulse and output a weighted pulsesignal having enhanced detectability, wherein portions of the inputsignal within the temporal duration are multiplied by a non-zerofunction and portions of the input signal that are beyond the temporalduration are rejected.
 2. The optical receiver of claim 1, wherein theoptical pulse generator comprises a four-wave mixer, wherein thecombination of the input signal and the optical weighting pulse isguided into a highly non-linear fiber (HNLF) to generate an idler. 3.The optical receiver of claim 1, wherein the optical bandpass filter isconfigured to remove a weighting pump signal and the input signal.
 4. Anoptical receiver for ultrashort signal detection, the receivercomprising: an optical pulse generator configured to generate an opticalweighting pulse that closely matches at least one ultrashort pulsewithin an input signal received by a direct detection receiver; and anoptical bandpass filter configured to filter the input signal and theoptical weighting pulse and output a weighted pulse signal havingenhanced detectability, wherein portions of the input signal that arewithin the temporal duration are multiplied by a non-zero function andportions of the input signal that are beyond the temporal duration arerejected.
 5. The optical receiver of claim 4, wherein the optical pulsegenerator comprises a four-wave mixer, wherein the combination of theinput signal and the optical weighting pulse is guided into a highlynon-linear fiber (HNLF) to generate an idler.
 6. The optical receiver ofclaim 4, wherein the optical bandpass filter is configured to remove aweighting pump signal and the input signal.
 7. A method for increasingsensitivity for detection of ultrafast optical signals, the methodcomprising: inserting an optical pulse generator upstream of a detectorwithin a direct detection receiver, wherein the optical pulse generatoris configured to generate an optical weighting pulse that closelymatches at least one ultrashort pulse within the input signal; andfiltering a combination of the input signal and the optical weightingpulse to output a weighted pulse signal having enhanced detectability sothat portions of the input signal that are nonoverlapping with the atleast one ultrashort pulse are rejected.
 8. The method of claim 7,wherein the optical pulse generator comprises a four-wave mixer, whereinthe combination of the input signal and the optical weighting pulse isguided into a highly non-linear fiber (HNLF) to generate an idler. 9.The method of claim 7, wherein filtering comprises removing a weightingpump signal and the input signal.